Materials:
big telescope, strong people, good eyes, decent artist, misc. photography materials
Introduction:
KeplerÕs three laws describe the motion and relationships of planetary orbit. Kepler's first law states that the orbits of the planets and other celestial bodies around the Sun are ellipses. An ellipse is defined as a figure drawn around two points called the foci such that the distance from one focus to any point on the figure back to the other focus equals a constant. (Please refer to page 99 in NS 121/123 reader) This constant is the measure of the long diameter of the ellipse, the major axis. Half of this segment is called the semimajor axis. The short diameter, the minor axis, is a perpendicular bisector of the major axis. Half of the minor axis is called the semiminor axis. For planets, the Sun is at one focus, nothing is at the other.
Kepler's second law states that a line from the planet to the Sun sweeps over equal areas in equal amounts of time. These areas in the ellipse are called sectors. In the following diagram, as the planet
moves from point A to point B along its orbit, a long, skinny sector is created.
If we wanted to create a sector of equal area at points closer to the Sun (points C and D), the result is a short, fat sector. According to Kepler, the time it takes for the planet to get from A to B is equal to the time it takes the planet to get from C to D. This means that a planet orbits slower as it moves further from the Sun.
Kepler's third law deals with the length of time a planet takes to orbit the Sun, called the period of revolution. The law states that the square of the period of revolution is proportional to the cube of
the planet's average distance to the sun: P2=a3. Because of the way a planet moves along its orbit, its average distance from the Sun is half of the long diameter of the elliptical orbit (the semimajor axis.) The period, P, is measured in years and the semimajor axis, a, is measured in astronomical units (AU), the average distance from the Earth to the Sun.
Proposed Procedure:
In order to test our hypothesis we plan to observe Jupiter and its four moons every other night for a month, and then for the rest of the experiment we will observe only once a week. When observing we will measure and note the distances between Jupiter and its moons. We also will map JupiterÕs position in the sky with drawings in relationship to our nightly observation of the moon. If possible, we also will be photographing our nightly observations of the Jovian system by focusing through the telescope with special exposure film. Hopefully, in doing so, we will attempt to support KeplerÕs theories.
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